When I fill out my W4 to take into account my new house, do I only multiply the monthly expenses (property tax and interest) by 10 months (April to December) or by the whole year? e.g If I claim 15 or 16 exemptions as calculated on an entire year, will it cause me to owe come tax time? Also, is the state tax that I have deducted out of my biweekly paycheck tax deductible when itemizing? Thanks. (I'm almost finished moving!!) :)